1. Field of the Invention
The present invention relates to laminated balun transformers, and more particularly, to a laminated balun transformer used as a balanced-unbalanced signal converter or phase converter in a radio communication IC chip.
2. Description of the Related Art
A balun transformer converts a balanced signal in a balanced transmission line into an unbalanced signal in an unbalanced transmission line and vice versa. xe2x80x9cBalunxe2x80x9d is an acronym from BALanced to UNbalanced. A balanced transmission line is provided with a pair of signal paths and a balanced signal transfers, as a voltage difference, between the two signal paths. In a balanced transmission line, since external noise equally affects the two signal paths, the external noise is canceled. Therefore, a balanced transmission line is unsusceptible to external noise. Since a circuit in an analog IC chip is configured with a differential amplifier, input and output terminals for an analog IC chip signal are of a balanced type which input and output signals as voltage differences between the two terminals, in many cases. In contrast, an unbalanced transmission line transfers an unbalanced signal, as a voltage, between one transmission line and the ground (zero voltage). It includes a coaxial line and a microstripline on a substrate.
A balun transformer, in which a winding is wrapped around a magnetic core such as ferrite in a bifilar winding, is conventionally used for a balanced-unbalanced converter in a transmission line of a high-frequency circuit. Such a balun transformer, however, has a large conversion loss in a high-frequency band above the UHF band and is limited as to size reduction.
In such a frequency band, a coaxial balun transformer 51 shown in FIG. 6 is used. The balun transformer 51 has a center electrode 55, one end of the center electrode 55 being connected to an input and output terminal 52a and the other end being made open. Around the center electrode 55, two internal electrodes 56a and 56b are provided so as to electromagnetically couple with the center electrode 55. The other two input and output terminals 52b and 52c are connected to the internal ends opposing each other of the two internal electrodes 56a and 56b through leads 57a and 57b, respectively. A ground electrode 58 is provided around the two internal electrodes 56a and 56b with a dielectric member disposed therebetween. Both ends of the ground electrode 58 are connected to the external ends of the internal electrodes 56a and 56b. 
Another balun transformer has also been proposed. This balun transformer is a laminated balun transformer 60 shown in FIG. 7. The balun transformer 60 includes a dielectric layer 61b on which a lead electrode 62 is provided, a dielectric layer 61c on which a xcex/2 stripline 63 is provided, a dielectric layer 61d on which xcex/4 striplines 64 and 65 are provided, and dielectric layers 61a and 61e on which ground electrodes 66 and 67 are provided, respectively. The xcex/4 striplines 64 and 65 are electromagnetically coupled with the left section 63a and the right section 63b of the xcex/2 stripline 63, respectively.
Since the balun transformer 51 of FIG. 6 has a coaxial structure, it is difficult to make it compact. Therefore, it is not suited to units such as mobile radio equipment which require a compact balun transformer.
Although the balun transformer 60 of FIG. 7 is definitely more compact than the balun transformer 51 having the coaxial structure, since the xcex/2 stripline 63 is routed on the dielectric layer 61c, the balun transformer 60 occupies a large area on a printed circuit board when it is mounted on the printed circuit board.
To adjust the electric characteristics of the balun transformer 60, electromagnetic coupling between striplines is adjusted by changing the thickness of a dielectric layer and the width of a stripline. However, there is no other way but to change the width of the xcex/4 stripline 64 or the line width of the left-hand section 63a of the xcex/2 stripline 63 to, for example, independently adjust electromagnetic coupling between the xcex/4 stripline 64 and the left-hand section 63a of the xcex/2 stripline 63 and electromagnetic coupling between the xcex/4 stripline 65 and the right-hand section 63b of the xcex/2 stripline 63. This is because, when the thickness of the dielectric layer 61c disposed between the xcex/4 striplines 64 and 65 and the xcex/2 stripline 63 is changed, electromagnetic coupling between the xcex/4 stripline 65 and the right-hand section 63b of the xcex/2 stripline 63 is affected. Adjustment by stripline width causes a slight change and it is not easy to adjust electromagnetic coupling between striplines.
Accordingly, it is an object of the present invention to provide a laminated balun transformer which allows easy adjustment of electromagnetic coupling between striplines and which can be made compact.
The foregoing object is achieved in one aspect of the present invention through the provision of a laminated balun transformer including at least two pairs of striplines each stripline of a pair being electromagnetically coupled through a dielectric layer, the pairs of striplines being separated with a dielectric layer interposed between the pairs in a stacked structure.
The foregoing object is achieved in another aspect of the present invention through the provision of a laminated balun transformer including a first dielectric sheet with a first stripline located at one surface thereof, a second dielectric sheet with a second stripline electromagnetically coupled with the first stripline located at one surface thereof, a third dielectric sheet with a third stripline located at one surface thereof, a fourth dielectric sheet with a fourth stripline electromagnetically coupled with the third stripline located at one surface thereof, and an electrically connection electrically connecting the first stripline and the fourth stripline wherein the first, the second, the third, and tie fourth dielectric sheets are in a stacked relationship one above another in a laminated structure. The electrical connecting means includes external electrodes provided on side faces of the laminated member and via holes provided inside the laminated member.
According to the present invention, since at least two pairs of striplines electromagnetically coupled with a dielectric layer disposed therebetween are provided, the two pairs of striplines being stacked through a dielectric layer, each stripline being laminated to a dielectric layer without being disposed on the same dielectric layer as another stripline, and a balun transformer having a small area is obtained. In addition, since the thickness of a dielectric layer sandwiched by one pair of electromagnetically coupled striplines can be adjusted independently of the thickness of the dielectric layer sandwiched by the other pair of striplines, a laminated balun transformer in which electromagnetic coupling between striplines can be easily adjusted is obtained,